Drosophila melanogaster males perform courtship; females produce pheromones which cause them to be sexually attractive. The ability to perform courtship is determined by the presence of one X chromosome in parts of th central nervous system; for pheromone production, there must be two X chromosomes in tissues within the abdomen. Mutations in the Sex-lethal (Sx1) gene perturb sex determination and dosage compensation, two responses to the number of X chromosomes in cells, and affect the ability of diplo-X flies to perform and elicit courtship. This proposal describes the analysis of the effects of Sxl mutations on tissues which must be haplo- or diplo-X for sexual behaviors. Since the Sx1 mutations are temperature-sensitive, temperature-shift experiments will reveal times at which Sx1 gene function affects the nervous system and addominal tissues. In addition, the question of where the Sx1 gene acts in the nervous system can be answered by determining whether clones of mutant tissue in parts of the brain which must be haplo-X for courtship are necessary and sufficient for the performance of courtship by diplo-X flies. The effect of Sx1 mutations on pheromone-producing tissues can be ascertained by observing the responses of mutant males with olfactory defects to diplo-X flies with Sx1 mutations and to volatile compounds extracted from Sx1 flies. Another sexual behavior, courtship of young males, has not been well-characterized behaviorally. Young males produce pheromones which stimulate courtship; mature males, which are not sexually attractive, produce courtship-inhibiting pheromones and also perform rejection behaviors in response to courting males. By observing the behavior in response to males of various ages of mutant males with olfactory and visual defects, it will be possible to identify the sequence of changes which occur in the sex appeal of males as they age. These studies will eventually facilitate identification of genes which control these aspects of sexual maturation in males. The long-range goal of these experiments is to elucidate the genetic control of sexual behavior in Drosophila, with particular emphasis on behaviors which involve production or responses to sex pheromones. This work may eventually have practical for biological control. Although D. melanogaster is not an agricultural pest, its behavior is typical of non-social insects and it can thus be considered a model system.